The present invention relates to photopolymerizable compositions or more particularly to photopolymerizable compositions suitable for producing improved, high contrast waveguides. The photopolymerizable compositions of this invention include a nitrone compound component.
It is known in the art to produce optical waveguides to form interconnections between optical information processing devices, or connections between such devices and other optical communication links such as glass optical fibers. Waveguides may also be used to create passive optical devices such as splitters, combiners, couplers, routers and the like. In telecommunications applications, single-mode waveguide devices with densely packed features having extremely small dimensions are generally required. The transverse dimension of such waveguides may range from about 5 .mu.m to about 10 .mu.m, while the space between guides can be as little as 3 .mu.m. It is known in the art to produce waveguides with UV photopatterned polymeric materials. In this regard, see U.S. Pat. No. 4,609,252 which is incorporated herein by reference. The ability to print images having the required dimensions, contrast and transparency depends on a number of interacting variables. These include UV exposure level, exposure time, the chemical and physical characteristics of the composition used including the activity of selected photomonomers, the spectral response of photoinitiators and the properties of any inhibitors and antioxidants which may be present.
It has been determined that conventional materials known to those of skill in the art for producing waveguides do not provide the required contrast to obtain the most desired image features. This shortcoming is particularly evident when it is required to produce images which have differing densities across a substrate which bears a photopolymerizable composition. Composition exposure methods are also important. While exposure by direct laser writing is generally effective in writing features one at a time, the more economical approach is to expose the photopolymerizable composition through a mask. However, this latter method has been found to produce unwanted partially exposed, i.e. partially polymerized regions between the image areas. Conventional additives have not been effective at providing sufficiently high contrast to stop polymerization at the boundary of exposed and nonexposed regions. Unwanted intermediate polymer gels which cannot be removed by the developing process are the result.
It has now been found that by including a free-radical polymerization inhibiting molecule containing a nitrone group in the photopolymerizable composition, a dramatic improvement in contrast results which allows the desired images to be formed using a uniform exposure through a conventional exposure mask. Without being bound to a particular theory, it is believed that the nitrone acts in two ways to overcome the difficulties encountered with conventional formulations. In the usual case, oxygen dissolved in the monomers serves as a photopolymerization inhibiting agent. In the UV exposed regions, free radicals generated by the activation of the initiator are first scavenged by the oxygen present in those regions. However, when the oxygen is consumed, polymerization proceeds. Ideally, oxygen present in the unexposed regions halts the polymerization at the boundary of the exposed regions. In the case of dense structures, the oxygen can be nearly exhausted in the confined unexposed regions between the printed features which are only a few microns wide. Ultimately, the combined diffusion of both the free radicals and the inhibiting species causes unwanted polymerization to occur in the unexposed regions as well. As a result, polymer gels are formed which destroy the intended function of the device. The nitrone is a relatively large molecule which diffuses much more slowly than oxygen, and is not subject to a concentration dictated by reaching an equilibrium with ambient conditions, but can be present in any desired amount. Furthermore, the nitrones are extremely efficient radical scavengers compared to other organic compounds used for this purpose. It has also been unexpectedly found that a further advantage can be obtained from the fact that nitrones photobleach by a process which converts them to stable and non-reactive oxaziridines. These oxaziridines do not function as radical scavengers, and once created allow the polymerization to proceed uninhibited. The peak wavelength for this photobleaching can be adjusted by the choice of substituents on the nitrone. Thus the degree and speed of bleaching can be controlled. In its unbleached state the nitrone can be engineered to absorb light which would otherwise activate the photoinitiator, and therefore initially limit the population of radicals by the dual action of light absorption and radical scavenging. A balance between the radical scavenging and light absorption mechanisms can be adjusted. Ultimately, the combined effects of radical scavenging and absorption followed by bleaching result in marked improvements in effective contrast which allows fabrication of the desired highly dense or closely spaced structures.
Nitrone containing polymers have been previously employed to form waveguides via photobleaching with a UV light (see U.S. Pat. Nos. 5,219,710 and 5,176,983). In this instance the difference in refractive index between the nitrone containing nonimage areas and oxaziridine containing image areas is used to delineate the waveguide. Nitrone containing molecules have also been used as surface layers over a photoresist to enhance contrast again via photobleaching. The use of a surface layer to enhance contrast is often not a viable process for practical applications, particularly in those applications employing liquid monomer or other components which require very accurate control of layer thickness, such as is needed for the single-mode structures described herein. The present invention uses nitrones blended in the bulk of the polymerizable composition mixture, where both its photobleaching and its radical scavenging properties can be employed together to achieve a synergistic effect.